1. Field of the Invention
This invention relates to apparatus comprising a burner and conveying means defining an air passage for conveying air to the burner and a fuel passage for conveying fuel to the burner, the conveying means being adapted to control the rate of flow of fuel in accordance with the rate of flow of air to maintain a constant fuel to air ratio at the burner when the rate of flow of air is varied.
2. Prior Art
Apparatus of the kind referred to which is commonly used at the present time has a regulator which includes a valve member movable relative to a body of the regulator to open and close an orifice through which the fuel flows, a first diaphragm which is connected with the valve member and which separates two chambers connected with the air flow path upstream and downstream respectively of an orifice in the air flow path and a second diaphragm which is also connected with the valve member and which separates two further chambers in or connected with the fuel flow path at opposite sides of an orifice in the fuel flow path. One of the chambers connected with the air flow path is adjacent to one of the chambers connected with the fuel flow path and it is necessary to prevent leakage between these chambers, whilst allowing unimpeded movement of the valve member. Sliding seals which are capable of preventing significant leakage interfere with movement of the valve member and prevent the required fuel to air ratio being maintained at the burner. For this reason, regulators generally used at the present time have a third diaphragm which separates the adjacent air and fuel chambers and a central portion of which is sealed to the valve member. During use of these known regulators, the third diaphragm exerts on the valve member a force which is related to the difference in pressure between the adjacent air and fuel chambers. Particularly in a case where the pressure in the air chamber is substantially different from the pressure in the fuel chamber, this force prevents the predetermined air to fuel ratio being maintained at the burner when there are substantial changes in the air flow rate.
Numerous devices comprising three diaphragms connected with a common central member have been proposed but none of these proposals has solved the problem of maintaining a predetermined air to fuel ratio at a burner when large changes are made in the air flow rate. It will be understood that burners may be required to operate with a flow rate which is only one tenth of the maximum flow rate at which the burner can be operated. The problem is further compounded by considerable differences between the absolute pressures at which air and fuel are made available to a burner. For example, in the case of oil fuel, oil is generally supplied at a pressure in the region of 100 p.s.i.; whereas air is generally supplied at a pressure in the region of 1 p.s.i. In contrast with this, gaseous fuel is often available to the burner only at a pressure below that of the air supply.
Examples of devices having three diaphragms connected with a common central member are disclosed in U.S. Pat. No. 2,886,698 issued to Johnson et al May 19, 1959. This Patent discloses devices which respond to the establishment of a predetermined ratio between two fluid pressures by providing an output signal, for example operating an electrical switch. The device includes a venturi to provide a reference ratio which is the ratio of the pressure at which gas flows to the venturi and the pressure at the throat of the venturi. Two of the chambers are connected with the venturi so that these pressures are exerted on appropriate diaphragms. The two pressures which are to be compared are applied to two other chambers. In each device illustrated in the Johnson et al Patent, the diaphragms are very different in size. The devices disclosed in the Johnson et al Patent are not flow regulating devices and the sets of diaphragms disclosed would not be useful in solving the problem of accurate flow regulation, owing to the large difference in size between the two or three members of each set.
A further device comprising three diaphragms connected with a common central member has been proposed in French Pat. No. 2,264,999 of Fr. Sauter AG Fabrik Elektr. Apparate, published on 17th October 1975. This device is a fluid pressure amplifier and the member connected with the diaphragms is a valve for controlling the fluid pressure maintained in one chamber of the device. As explained in the French Specification, the device illustrated in FIG. 1 thereof is not satisfactory as an amplifier and the device would not solve the problem of accurate control of the rate of flow of fuel to a burner. The French Specification proposes that a device having only two diaphragms and two annular, flexible seals should be used to achieve improved results.
A device which is intended to control accurately the rate of flow of fuel to a burner is disclosed in U.S. Pat. No. 3,101,897 issued Aug. 27th, 1963 to Vaughn. The Vaughn device comprises three diaphragms connected with a valve by a tubular member which passes through the centres of the diaphragms. This tubular member provides direct communication between outer chambers of the device, one of these communicating with the fuel flow path at a position downstream of the valve orifice. Intermediate chambers of the device are subjected to respective pressures at spaced positions along the air flow path. Any variation in the back pressure in the fuel flow path between the flow regulator and the burner would prevent maintenance of the required air to fuel ratio at the burner. Similarly, any variation in the pressure at which fuel is supplied to the regulating device would also prevent the required ratio being maintained.
An earlier proposal for a device capable of maintaining the required relation between fuel and air flow rates to a burner is disclosed in British Pat. No. 120,076 of Keith, accepted Oct. 21st, 1918. The flow regulating device described in the Keith Patent has only two diaphragms, these being connected together to a valve in the fuel flow passage. A space between the diaphragms is divided by a rigid wall into chambers which communicate with one another via an adjustable orifice. These chambers are both in the fuel flow path. Outer faces of the diaphragms, which have equal areas, are connected with respective positions spaced apart along the air flow path. The Keith device will be in equilibrium only when the pressure drop between these positions in the air flow path is exactly equal to the pressure drop between the two chambers in the fuel flow path. However, equal pressure drops are generally unsatisfactory. Ideally, the pressure drop in the air flow path and the pressure drop in a gaseous fuel flow path are both preferably about 10% of the supply pressure. Since the air supply pressure is normally considerably greater than the gas supply pressure, maintenance of equal pressure drops in the two flow paths would prevent the use of 10% pressure drops in one or other of the flow paths. The problem is even more pronounced in the case of an oil burner, where the supply pressure of the oil is likely to be 100 times greater than the air supply pressure. Clearly, equal pressure drops in this case would be impracticable. At same value of the pressure drop in the oil flow path below 5 p.s.i., the flow would become laminar and the proportional relation between flow rate and the square of the pressure drop would be lost so that accurate control of the oil flow rate in accordance with the air flow rate would be impossible.